The invention relates to a process and a device for the generation of a measurement signal Mx, which is a linear function of the numerical density Nx of absorbing atoms or molecules in a sample volume, with a Zeeman atomic absorption spectometer, in which the spectral lamp generating the measurement beam is disposed in a magnetic field.
Various processes of Zeeman atomic absorption spectroscopy (ZAAS) are known. A brief survey of the processes which are used in practice can be found in "Nachrichten aus Chemie, Technik und Laboratorium" (Reports from the Chemical, Engineering and Laboratory Sectors), volume 29, No. 12/81. The invention is concerned with direct ZAAS, in which the emission line is split into the two polarization components .pi. and .sigma.. The .pi. component is employed to measure the atomic absorption and the background absorption, and the .sigma. component is employed to measure only the background absorption closely adjacent to the atomic absorption. The pure atomic absorption measurement signal is thus obtained by difference formation.
In the known measuring arrangements, the difference of the transmitted intensities I.pi. and I.sigma. is used as the measurement signal. It is known that the difference signal is an approximately linear function of the number of absorbing atoms or molecules only up to a specific sample concentration. After reaching a maximum, the difference signal decreases again if there is a further increase in the sample concentration.
This phenomenon is described as the "roll-over effect" of the measurement curve and is explained by a more or less intense specific absorption of the .sigma. component as well.
For both components .pi. and .sigma., the transmitted intensity is a function of the irradiated intensity Io and the absorption coefficient .alpha., according to the absorption law I=Io.multidot.e.sup.-.alpha..multidot.d. The absorption coefficient is proportional to the numerical density N of the absorbing atoms (or molecules), and "d" is the thickness of the absorbing layer. If the layer thickness and absorption constant are now combined, then for the two transmitted intensities the result is I.pi.=I.pi.o.multidot.e.sup.-.alpha..pi..multidot.N and I.sigma.=I.sigma.o.multidot.e.sup.-.alpha..sigma..multidot.N.
For the measuring arrangements which are customarily employed, it can be assumed that I.sigma.o-I.pi.o=Io and .alpha..pi.&gt;.alpha..sigma.. If N is now varied from 0-&gt;.infin., then this results in measurement curves for I.pi. (N) and I.sigma. (N) which commence at Io and tend towards 0 in the form of an e function as N becomes greater. In the known measuring arrangements, the difference of the transmitted intensities (I.pi.-I.sigma.) is evaluated as the measurement signal. This measurement signal commences at 0 (I.sigma.o-I.pi.o=0), then increases with increasing N because .alpha..pi.&gt;.alpha..sigma., and then again tends towards 0, since both I.pi. and also I.sigma. tend towards 0 as N-&gt;.infin..
The object of the invention is to provide a process and a suitable measuring device which are suitable to expand the measurement range of ZAAS by linearization of the calibration curve into the region of higher sample concentrations.